Title

Author

Date of Award

Level of Access

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Jasmine E. Saros

Second Committee Member

Ann Dieffenbacher-Krall

Third Committee Member

Kevin Simon

Abstract

Nitrogen (N) limited alpine lakes have been receiving enhanced atmospheric N deposition over the last century. This increase has elicited dramatic changes in diatom community structure, but the surface water concentrations of nitrate that cause this shift remain unclear. We attempted to quantify these concentrations by developing a diatom calibration set using surface sediment samples from 46 lakes across the U.S. Rocky Mountain Range. Out of the sixteen measured environmental variables, ordination analysis identified that nitrate, conductivity, total phosphorus, and temperature are related to diatom distributions. A transfer function was developed for nitrate and applied to a suite of sedimentary diatom profiles from lakes across the Rockies. The model performance coefficient (bootstrapping validation) of 0.610 suggested high potential for robust diatom-inferred reconstructions lakewater nitrate concentrations over time. However, comparisons of diatom-inferred nitrate to historical nitrate values revealed very poor performance of the model, with diatom-inferred values almost always much higher than observed ones. The frequent domination of diatom assemblages by three generalist species (Discostella stelligera, Achnanthidium minutissimum, Staurosirella pinnata) is likely the key problem with this model. This complication underscores the utility of the indicator species approach in understanding the effects of nitrate enrichment in oligotrophic lake alpine lakes by focusing ecologically relevant species.